1. Field of the Invention
The present invention relates to an opto-acoustic apparatus and a method for using it for measuring the concentration of gas in a mixture of gases and the apparatus comprises providing an opto-acoustic cell into which a mixture of gases including the gas the concentration of which is to be measured is introduced, directing laser rays from a laser device for directing laser rays into said opto-acoustic cell, placing a chopper in the path of the laser rays for chopping the laser rays directed from said laser device, and placing a microphone in the cell for detecting the sound signal generated by the changes in the internal pressure in said opto-acoustic cell and determining the concentration of the gas from the sound signal.
2. Description of the Prior Art
There has heretofore been provided an apparatus for measuring the concentration of solid molecules in a gas introduced into an opto-acoustic cell, e.g., the concentration of particulates, such as smoke, in the exhaust gas of internal combustion engines and the like, and the concentration of specific gas molecules in a mixture of gases, e.g., the concentration of gas molecules such as ammonia, ethylene, and ozone in air, by utilizing an opto-acoustic effect.
An example will be described of the measurement of the concentration of particulates in the exhaust gas of an automobile engine. When laser rays are directed into the exhaust gas of such an engine, which exhaust gas has been introduced into an opto-acoustic cell and contain particulates therein, said particulates absorb optical energy and are heated. If said laser rays are chopped, said particulates are repeatedly heated and cooled in accordance with the chopping frequency, whereby a change of pressure, that is to say a change of acoustic pressure, is produced in said cell.
This change of acoustic pressure produces a sound wave, hereinafter referred to as a sound signal, which has a frequency equal to the chopping frequency of said laser rays. The strength of said sound signal is proportional to the amount of optical energy absorbed by said particulates. Thus, the concentration of said particulates can be measured by detecting said sound signal by means of a microphone and determining the strength thereof.
However, since said sound signal which is generated in said opto-acoustic cell is very weak, it has been necessary to use a highly sensitive microphone to detect said sound signal into a suitable electric signal, at present state it is necessary to use an expensive narrow band amplifier or a synchronizing rectifying amplifier, or both, for said amplifier means, since extraneous sounds, such as noise, may be large in comparison with said sound signal.
This is particularly true in the measurement of the concentration of particulates contained in the exhaust gas of an automobile engine. In such a case, exhaust noises are transmitted to said cell together with the exhaust gas. Accordingly, a muffler means such as a buffer tank is installed in the exhaust feed passage. In addition, a muffler means such as buffer tank is also installed between said cell and a suction pump for preventing noise from being transmitted to said cell from said suction pump which draws gas into said cell.
It is, however, necessary to use an expensive large-scale muffler means in order to sufficiently reduce exhaust noises and suction noises. Besides, there are such defects that there is delay in the introduction of a sample, particulates tend to collect in said muffler means, and the like. These defects lead to errors in measurement, whereby there are natural limits to the effectiveness of said muffler means. Moreover, when there are noises more intense than the allowable range of said microphone and said amplifier, measurement becomes impossible.